This invention relates to a sealing glass having a high coefficient of absorption for infra-red rays which is particularly useful for making the envelope of a switch with flexible metal blades; this envelope being sealed or welded tightly through the application of infra-red rays.
The fabrication of switches with flexible metal blades or reeds (also called reed switches) is conducted in a closed chamber in an appropriate gaseous atmosphere (reducing or neutral) or in a vacuum. Glass tubes are utilized for making the envelope of the switches and the sealing of each tube to the blades of each switch is effected automatically by a mechanical apparatus with infra-red radiation being applied to the ends of each tube in such a manner as to heat the glass and bring it to a temperature sufficient to seal the blades tightly. The application of infra-red radiation is generally accomplished by using the classic halogen lamp as the source of infra-red radiation, mirrors reflecting the said radiation onto the tube to be sealed.
To be suitable for such an application, the glass must have a certain number of particular properties:
(a) its coefficient of thermal expansion must be compatible with that of the metal constituting the metal contacts; PA0 (b) it must exhibit adequate thermal and mechanical resistance; PA0 (c) it must exhibit a high coefficient of absorption, i.e., greater than 98%, for infra-red radiations within the band of wavelengths ranging approximately from 0.7 to 4 microns; PA0 (d) it must exhibit a low rate of volatilization at the sealing temperature in order to minimize fouling of the apparatus, particularly the reflecting mirrors, and contamination of the metal blades inside the switch itself; and PA0 (e) it must exhibit a relatively low sealing temperature to reduce the amount of energy to be added for securing the sealing or welding.
A commercial glass commonly used for such an application is sold by Corning Glass Works under Code No. 9365 in its catalog. The composition of this glass, in weight percent, consists of about:
______________________________________ SiO.sub.2 70.38% Al.sub.2 O.sub.3 2.90 Li.sub.2 O 0.68 Na.sub.2 O 8.85 -K.sub.2 O 6.20 BaO 7.75 F 0.64 Fe.sub.2 O.sub.3 2.90 ______________________________________
This glass, however, has a rate of volatilization on the order of 320 ppm and a temperature at 10.sup.4 poises on the order of 1000.degree. C. and it appeared desirable, in order to meet the desires of the customers, to develop a glass having a lower rate of volatilization and sealing temperature.
Accordingly, it is the object of the invention to provide such a glass.